public class Solution {
    public long prev = Long.MIN_VALUE;

    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }
        if (!isValidBST(root.left)) {
            return false;
        }
        if (root.val <= prev) {
            return false;
        }
        prev = root.val;
        if (!isValidBST(root.right)) {
            return false;
        }
        return true;
    }
}
class TreeNode {
     int val;
     TreeNode left;
     TreeNode right;
     TreeNode() {}
     TreeNode(int val) { this.val = val; }
     TreeNode(int val, TreeNode left, TreeNode right) {
         this.val = val;
         this.left = left;
         this.right = right;
     }
 }

